<button onclick='send()'>Pobierz aktualne pliki csv</button>
<script>
  var send = function() {
      var items = ['731','421'] //pobieranie z listy
for(var i=0, l = items.length; i < l; i++){
    var request = new XMLHttpRequest();
request.open('POST', 'https://www.iaai.com/Auctions/BranchListingView.aspx?branchCode=' + items[i], /* async = */ false);
var formData = new FormData();
    formData.append('__EVENTTARGET', 'ctl00$ContentPlaceHolder1$DownloadLinkButton');
request.send(formData);
console.log(request.response);

let csvContent = "data:text/csv;charset=utf-8," + request.response;
var new = csvContent.replace('#', '');
var encodedUri = encodeURI(new);
window.open(encodedUri);
}

  };
</script>